Tandem mass spectrometers are well-established tools for solving an array of analytical problems. Common analytical problems involve liquid phase samples. Some ion source types, such as electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), or inductively coupled plasma (ICP), operate at or near atmospheric pressure. These are readily coupled to separation methods such as Gas Chromatography (GC), Liquid Chromatography (LC), Capillary Electrophoresis (CE) and other solution sample separation systems. However, most mass spectrometers operate at pressures substantially below atmospheric pressure. In such cases, the ions must be transferred from a high-pressure region to a lower pressure region.
Conventionally, electrically isolated apertures are used to separate adjacent pressure regions. Voltages are applied to the apertures to focus ions into adjacent vacuum regions. Ion losses occur during ion transfer due to scattering of ions against background neutral gas. As taught by Whitehouse et. al. in U.S. Pat. No. 5,652,427 and U.S. Pat. No. 6,011,259, which is fully incorporated herein by reference, one method that overcomes such problems involves transporting ions through RF multipole ion guides that extend between vacuum regions. The RF multipole ion guides are configured with an appropriate diameter to serve as conductance limiting elements, replacing the electrically isolated apertures.
Pressurized RF multipole ion guides have been used to achieve damping of ion kinetic energy during ion transmission from Atmospheric Pressure Ionization (API) Sources to mass analyzers. Ion collisions with the neutral background gas reduce the primary ion beam kinetic energy spread. Ion transmission efficiency through the ion guide and downstream of the ion guide is improved. Additionally, because the ion energy spread is low, the apparent resolving power of quadrupole mass analyzers is improved. A quadrupole ion guide, operated in RF only mode in the presence of increased background pressures, is taught by Douglas et. al. in U.S. Pat. No. 4,963,736.
An important application of tandem mass spectrometers is the identification of molecular ions and their fragments by mass spectrometric analysis (MS and MS/MS, respectively). A tandem mass spectrometer performs molecular ion identification performed by mass-selecting a precursor ion of interest in a first stage, fragmenting the ion in a second stage, and mass-analyzing the fragment in a third stage. Tandem MS/MS instruments are either sequential in space (for example, consisting of a two quadrupole mass filters separated by a collision cell) or sequential in time (for example, a single three-dimensional ion trap). Commercial three dimensional ion traps perform multiple stages of fragmentation (MS/MSn). Currently existing commercial tandem mass spectrometers typically perform one stage of fragmentation (MS/MS).
Whitehouse et. al. in U.S. Pat. No. 5,652,427 describe a hybrid mass spectrometer wherein at least one multipole ion guide is configured with a Time-Of-Flight mass analyzer, which is fully incorporated herein by reference. As described, at least one quadrupole ion guide can be operated in ion transmission, ion trapping, mass to charge selection and/or collision induced dissociation (CID) fragmentation modes or combinations thereof coupled with Time-Of-Flight mass to charge analysis. In an improvement over the prior art, Whitehouse et. al. in U.S. provisional application Ser. No. 09/322,892, which is fully incorporated herein by reference, describe multiple quadrupole ion guides operated in a higher pressure vacuum region of a hybrid TOF mass analyzer, improving the mass analyzer performance and extending the analytical capability of a hybrid TOF mass analyzer. The hybrid quadrupole Time-Of-Flight apparatus and method described allows a range of MS, MS/MS and MS/MSn to be performed in the RF multipole ion guide configuration.
In the prior art, RF multipole ion guides are configured adjacent, end-to-end, to other multipole ion guides which also extend through various vacuum regions. The pressure within the multipole ion guides reduces continuously along the ion path, creating a pressure gradient. Each subsequent RF multipole ion guide operates in a region of reduced pressure from the previous one. This prior art configuration provides the ability to perform a range of MS, MS/MS and MS/MSn at elevated pressure. As an extension of these embodiments, increased analytical functionality can be achieved by operating a mass analyzer in a low-pressure region for MS followed by another high pressure region for MS/MS.
For example, it is sometimes preferable to perform mass selection utilizing an RF/DC resolving quadrupole resolving quadrupole, which routinely operate at low pressure. RF/DC resolving quadrupoles are the most commonly used mass filters for tandem mass spectrometers, because they are easy to use, they are very stable, and they provide suitable resolving power and sensitivity. As will be described below, RF/DC resolving quadrupole resolving quadrupoles require sufficiently low pressure that the ions undergo few or no collisions with background gas molecules.
Conventionally, the RF/DC resolving quadrupole quadrupoles are followed by a higher pressure RF multipole collision cell in which precursor ions undergo CID. RF multipole ion guides are used as collision cells for MS/MS in tandem MS/MS instruments. At elevated pressure, they efficiently contain the fragments produced by collision induced dissociation (CID). They are used as collision cells for the CID fragmentation of ions in triple quadrupoles, hybrid magnetic sector and hybrid TOF mass analyzers. Usually fragmentation is induced using an accelerating DC potential. RF multipole ion guide collision cells have been incorporated in commercially available mass analyzers. Commonly, they are configured as individual ion guide assemblies with a common RF applied along the collision cell multipole ion guide length. Quadrupole ion guides and ion traps have been configured as the primary elements in single and triple quadrupole mass analyzers and as part of hybrid mass spectrometers that include Time-Of-Flight, Magnetic Sector, Fourier Transform and three dimensional quadrupole ion trap mass analyzers.
Most commonly, quadrupole ion guides with RF/DC resolving quadrupole applied to either set of pole pairs are used. The well-known equations of ion motion in a quadrupole ion guide are described by Dawson, Chapter II of “Quadrupole Mass Spectrometry and Its Applications”, Elsevier Scientific Publishing Company, New York, 1976. The first stability region is determined by the solution of the Mathieu parameters q and a where:a=ax=−ay=4zU/mΩ2r02  (1)q=qx=−qy=2zV/mΩ2r02  (2)
U is the +/−DC amplitude, m is the ion mass, z is the ion charge, V is the RF (peak-to peak) amplitude, ro is the distance from the centerline to the quadrupole rod inside surface and Ω (=2πf) is the angular frequency of the applied RF field. Solutions for the equations of motion are plotted along iso-β lines as a function of q and a. Only those ions having mass to charge values that fall within operating stability region have stable trajectories in the x and y (radial) directions during ion trapping or ion transmission operating mode in a quadrupole ion guide. In low vacuum pressure quadrupole ion guide operation, mass to charge selection is typically conducted by operating near the apex of stability region where a=0.23699 and q=0.70600. The stability coefficient β can be expressed in simple terms of a and q for q<0.4, and β<0.6:β=(a+q2/2)1/2  (3)
A more accurate definition of β, appropriate for q>0.4 and β>0.6, given in terms of an expansion in a and q, is provided in the text by Dawson.
Typically, resolving RF/DC quadrupole ion guides are operated in background vacuum pressures that minimize or eliminate ion to neutral background gas collisions. Collisions within the RF/DC resolving quadrupole ion guide change the phase space of the ion, causing the ion to be ejected from the region of stability, and dramatically reduce the transmission efficiency. As noted by Dawson, ions with mass to charge values that fall close to the stability diagram boundary increase their magnitude of radial oscillation. As the resolving power of the RF/DC quadrupole is increased, those ions with phase space coordinates outside an acceptable limit are ejected and strike the rods. This effect is worse at elevated pressures.
A second mass- to-charge selection mode uses a range of auxiliary excitation frequencies in combination with RF or RF/DC to reject unwanted ions. Unlike resolving RF/DC quadrupoles, in this mode several mass-to-charge values can be transmitted simultaneously. Thus this approach can increase the speed of an analysis. Additionally this approach performs suitably at elevated pressure, unlike RF/DC quadrupoles. Numerous approaches using this mode have been developed for three dimensional ion traps, as described by Wells et. al. in U.S. Pat. No. 5,608,216, and references therein. For example, Wells describes an approach whereby a set of auxiliary frequencies is applied to a three dimensional ion trap to eject unwanted ions, and the RF is scanned over a small range of voltage to modulate the ion secular frequency, bringing it into resonance with the applied auxiliary frequency.
Auxiliary excitation is usually performed using dipolar or quadrupolar excitation, and can be performed with or without +/−DC applied the rods. When no DC is applied, the x and y component of the secular motion are identical; there is no differentiation between the A pole (where +DC is applied) and B pole (where −DC is applied). When resolving DC is applied, the ion motion in the x direction moves to higher frequency, and the motion in the y direction moves to lower frequency, and eventually at the apex of the stability diagram βx˜1 and βy˜0. In general, the fundamental ion motion (n=0) is given byωo=βΩ/2  (4)which can be expressed in terms of a and q for β<0.6 by the relation:ω0=(au+qu2/2)1/2Ω0/√2  (5)
Higher order components, expressed in terms of β, are:ω−1=(1−β/2)Ω for n=−1  (6)ω+1=(1+β/2)Ω for n=+1  (7)ω−2=(2−β/2)Ω for n=−2, etc.  (8)
In dipolar excitation, an auxiliary voltage typically is superimposed on one pole of a pair (the A pole or the B pole) while the other pole is referenced to ground. For dipolar excitation, the fundamental resonance n=0 is excited at or near
            ϖ      x        =                            β          x                ⁢        Ω            2        ;
      ϖ    y    =                    β        y            ⁢      Ω        2  
Thus dipole excitation applied along the A-pole results in a notch in ωx, and applied along the B-pole, a notch in ωy. For a=0, βx=βy and therefore:
                              ϖ          x                =                              ϖ            y                    =                      βΩ            2                                              (        9        )            
The subsequent ion motion is driven along the direction of the resulting dipole. When dipole excitation is applied to both pairs of rods (the A pole and the B pole), the ion motion is directed along some angle between the rods, depending on the selected phase between the two dipoles. The direction of ion motion can be determined by the vector sum of the forces along each axis. At a phase of 90°, the ion motion rotates about the axis, and this rotation can be useful in cases where it is desirable to prevent the ion from crossing the axis. Additionally, the ion energy is much more uniform than the other trajectories, where there is a large variation in energy due to the large periodic variations in radial amplitude.
For quadrupolar excitation, an additional, small amplitude quadrupolar voltage is superimposed on the larger amplitude quadrupolar voltage that is applied to the A and B poles:VA=C′ sin(2ω′t+φ) and  (10)VB=C′ cos(2ω′t+φ)
Sudakov, et. al discussed in detail the theoretical basis for the resonance structure (JASMS, 1999, 11, 10). The most efficient excitation occurs for resonances for n=1 and K=1 at frequencies:
                                                        2              ⁢                              ϖ                x                                                    K              =              1                                =                                                                      (                                      1                    ±                    β                                    )                                x                            ⁢              Ω                                                                            ;                                            2              ⁢                              ϖ                y                                                    K              =              1                                =                                                    (                                  1                  ±                                      β                    y                                                  )                            ⁢              Ω                                                                                                (        12        )            where the secular frequency is still defined as ωx and ωy. Rearranged, this gives the resonances for quadrupolar excitation:for a≠02ωx,Ω−2ωx,Ω+2ωx  (13)ωx,Ω−2ωx,Ω+2ωx  (14)and for a=02ω,Ω−2ω,Ω+2ω  (15)
In the simplest case excitation can occur at three distinct frequencies. The ion motion obtained by quadrupolar excitation is determined by the original position and momentum of the ion as it enters the quadrupole. Unlike dipole excitation there is no forced directionality. Thus the set of ions undergo a wide spread of trajectories. Commonly a is set to 0, and either dipolar excitation is used, exciting ω0, or quadrupolar excitation is used, exciting 2ω0, Ω−20, or Ω+2ω0. Providing a small value of a permits better definition of the low q stability edge and improved definition of the high mass cut-off point.
Dipolar excitation is sometimes preferable to quadrupolar excitation, in part because of the fewer number of resonances, and in part because the ion motion is readily controlled, since the ion is driven along the axis of the applied dipole rather than moving with the quadrupolar field. In some applications, dipolar and quadrupolar excitation is used simultaneously in order to take advantage of the different range of excitation frequencies, the different trajectory patterns, or the different rates of radial excitation. Franzen (US Patent, check) utilized combinations of dipolar and quadrupolar excitation in three dimensional traps. Additionally, quadrupole electrode structures can be constructed to contribute a small fraction of higher order field components to the primarily hyperbolic field, as described for three dimensional ion traps permitting an alternative method to affect the rate of radial excitation and ejection.
Although the radial excitation techniques described above are often performed at elevated pressure In ion guides or traps, the mass selectivity for continuous beams is superior at reduced pressure. At elevated pressure, the ion experiences collisional damping caused by energy loss due to momentum changing collisions with the background gas. The amplitude used for excitation must be increased to accommodate the energy loss due to collisions. High amplitude excitation yields poorer selectivity than low amplitude excitation for the same secular frequency, due to excitation of off-resonant frequencies near the secular motion of the ion.
As is also well known in the art, a third mass-to-charge selection mode for rejection of ions at some m/z values and selection of others is the use of high-q, low mass cut-off and low-q, high mass cutoff. Often a small amount of +/−DC is applied to the rods to enhance the definition of the stability edge, particularly for low-q. Here too the mass selectivity is best when the ion encounters few or no collisions.
Therefore, this invention is an extension of the prior art described in U.S. patent application Ser. No. 09/322,892, where the multiple RF multipole ion guides are positioned end-to-end along a continuously dropping pressure. In particular, the prior art does not provides means for low pressure mass-to-charge selection followed by high pressure CID. The present invention comprises multiple RF multipole ion guides, positioned end-to-end, with pressure suitably low in one RF multipole ion guide to provide functions such as mass-to-charge selection, followed by pressure suitably high in another RF multipole ion guide, to provide functions such as CID, and with multiple RF ion guides that extend between the various pressure regions, replacing electrostatic apertures.
Quadrupole ion guides, as described by Brubaker in U.S. Pat. No. 3,410,997, Thomson et. al. in U.S. Pat. No. 5,847,386 and Ijames, Proceedings of the 44th ASMS Conference on Mass Spectrometry and Allied Topics, 1996, p 795 have been configured with segments or sections where RF voltage generated from a single RF supply is applied to all segments of the ion guide assembly or rod set. Ijames describes operating the quadrupole assembly in RF only ion transport and trapping mode. The offset potential applied to segments of an ion guide can be set to trap ions within an ion guide section or segment as well. Douglas in U.S. Pat. No. 5,179,278 describes a quadrupole ion guide configured to transmit ions from an Atmospheric Pressure Ionization (API) source into a three dimensional quadrupole ion trap. The quadrupole ion guide described by Douglas in U.S. Pat. No. 5,179,278 can be operated as a trap to hold ions before releasing ions into the three dimensional quadrupole ion trap. During ion trapping, the potentials applied to the rods or poles of this quadrupole ion guide can be set to limit the range of ion mass to charge values released to the ion trap. The quadrupole ion guide can also be operated with resonant frequency excitation for collisional induced dissociation fragmentation of trapped ions prior to introducing the trapped fragment ions into the three dimensional ion trap. After the quadrupole ion guide has released all its trapped ion population to the three dimensional ion trap, it is refilled during the three dimensional ion trap mass analysis time period. Dresch et. al. in U.S. Pat. No. 5,689,111, which is fully incorporated herein by reference, describe a hybrid multipole ion guide Time-Of-Flight (TOF) mass spectrometer wherein the multipole ion guide is configured and operated to trap ions and release a portion of the trapped ions into the pulsing region of the TOF mass analyzer.
A conventional instrument configuration for tandem MS/MS and CID uses RF multipole ion guides for mass analysis. FIG. 1 illustrates a conventional triple quadrupole mass spectrometer. In conventional triple quadrupole mass analyzers, as shown in FIG. 1, single mass to charge range is selected in the first analytical quadrupole by applying appropriate RF and +/−DC potentials to the quadrupole rods. This is also the case for hybrid quadrupole TOF mass analyzers, where the third quadrupole in a triple quadrupole has been replaced by a TOF mass analyzer. Other mass analyzers, such as three dimensional ion traps, hybrid magnetic sector and Fourier Transform (FTMS) mass analyzers, also have been configured to perform MS/MS analysis. CID in triple quadrupoles and hybrid quadrupole-TOF mass analyzers is achieved by acceleration of ions along the quadrupole axis into a collision cell referred to herein as DC acceleration CID fragmentation. Ions are generally accelerated with a few to tens of eV in quadrupole DC acceleration CID fragmentation. Ion traps and FTMS mass analyzers perform MS/MSn analysis, however, ion CID fragmentation is performed with relatively low energy resonant frequency excitation. Hybrid or tandem magnetic sector mass analyzers can perform high energy DC acceleration ion fragmentation with ions accelerated into collision cells with hundreds or even thousands of electron volts.
Conventionally, in a mass spectrometer that must transport ions through multiple vacuum stages from atmospheric to low pressure, electrostatic lenses with small apertures are positioned between the moderate and low vacuum chambers to permit differential evacuation as well as ion transport into the low pressure region. Typically, a first RF multipole ion guide is operated in a moderate pressure region (1-100 mtorr), substantially reducing the kinetic energy spread and angular distribution of the ions. However, as the ions are focused through the electrostatic aperture, their energy and angular distribution becomes perturbed by collisions. Conventionally, in the lower pressure vacuum stage, the ions are then transported through the RF plus +/−DC quadrupole ion guide for mass to charge selection. However, scattering collisions encountered through the electrostatic lenses prior to entering the RF plus +/−DC resolving quadrupole increases the phase space of the ion beam, reducing its compatibility to the phase space entrance requirements. Therefore sensitivity and resolving power are reduced. Conventionally, commercially available mass spectrometers use RF Brubaker lenses in between the electrostatic lens and the resolving quadrupole in an attempt to recover losses. Similarly, CID is often performed in an RF multipole collision cell that is enclosed by electrostatic apertures. Ions are accelerated into a high pressure region through the first electrostatic aperture. The subsequent fragment ions are extracted out of the RF multipole collision cell by the second electrostatic aperture. Scattering collisions are again encountered, reducing the transmission of the ion beam as well as increasing the phase space of the beam, making it less compatible for the final mass analyzer.
A diagram of the multipole ion guide configuration of a conventional triple quadrupole mass analyzer 1 interfaced to Atmospheric Pressure Ion source 2 is shown in FIG. 1. Individual multipole ion guide assemblies 3, 4, 5 and 6 are aligned along the same centerline axis in a three stage vacuum pumping system. Capillary 7 provides a leak from atmospheric pressure Electro spray ion source 2 into first vacuum pumping stage 8. Ions produced in Electro spray source 2 are transferred into vacuum through a supersonic free jet expansion formed on the vacuum side of capillary exit 9. A portion of the ions are directed through the including orifice in skimmer 10, multipole ion guide 3, the orifice in electrode 11, multipole ion guide 4, the orifice in electrode 12, multipole ion guide 5, the orifice in electrode 13, multipole ion guide 6, the orifice in electrode 14 to detector 15. The pressures in vacuum stages 8, 16 and 17 are typically maintained at 0.5 to 4 torr, 1 to 8 millitorr and <1×10−5 torr respectively while the pressure inside collision cell 18 is maintained at 0.5 to 8 millitorr. Triple quadrupoles are configured to perform MS or a single MS/MS sequence mass analysis functions. In an MS/MS experiment, ions produced at or near atmospheric pressure, are transported through multiple vacuum stages to the low pressure vacuum region 17 where mass to charge selection occurs in quadrupole 4 with little or no ion to neutral collisions. Mass to charge selected ions are then accelerated through an electrostatic aperture into a region of elevated pressure in collision cell multipole ion guide 5. The resulting fragment ion population is extracted through yet another electrostatic aperture and is directed into quadrupole 6 residing in low pressure vacuum region 17. Mass to charge selection is conducted on the ion population traversing quadrupole 6 with few or no ion to neutral collisions prior to detection of stable trajectory ions exiting quadrupole 6 by ion detector 15. Quadrupole 4 is configured with RF only sections 19 and 20 at its entrance and exit end respectively. Quadrupole 6 is shown with RF only section 21 at its entrance. In commercially available hybrid quadrupole TOF mass analyzers quadrupole 6 is replaced by a TOF mass analyzer residing in a fourth vacuum pumping stage. Commonly, in this case the ions are extracted directly from collision cell 5, using electrostatic apertures and grid lenses, into the TOF.
The invention disclosed herein is an improvement over the prior art described in FIG. 1. In FIG. 1, electrodes 11, 12 and 13 are used extract ions from a higher pressure region to low pressure region 17. These incur sensitivity losses due to scattering. In this invention, an RF multipole ion guides replaces the differential pumping aperture into an RF/DC resolving quadrupole. This preserves the phase space of the ion beam, and improves the resolution-transmission characteristics of the resolving mass analyzer.
In this invention, multipole ion guides replace the differential pumping apertures within the collision cell, and are of sufficient diameter to limit conductance through the collision cell entrance and exit. The invention herein greatly reduces scattering losses that occur due to extraction of the ion beam from collision cell 5, and preserves the ion beam quality.
It is important to have a well-defined beam, of low radial divergence, for mass analysis by the TOF. In the example in FIG. 1, ions are extracted from collision cell 5 into the TOF, using electrostatic apertures and grid lenses. In the invention disclosed herein, an RF multipole ion guide is configured to extend between a high pressure region of the RF multipole collision cell and one or more low pressure regions adjacent to the entrance of a TOF, or other mass analyzers. Thus ions are smoothly transported out of collision cell 5 and into the lower pressure regions by use of the exit RF multipole ion guide, with few scattering losses. Similarly this invention provides the ability to decouple the extraction of ions from the higher pressure collision cell from the process of ion transport into the TOF, or other mass analyzer region, providing a well-defined beam with appropriate phase space conditions following the collision cell.
Finally, this invention provides additional forms of CID. For example, CID can be achieved by accelerating the ions in regions of pressure gradients. In particular it is possible to induce fragmentation in the RF multipole ion guide a portion of which is positioned in the collision cell. In this case the ions can fragmented in a higher pressure region, near the exit of the collision cell, but only undergo one or two collisions with substantially little cooling thereafter. In such cases there can be reduced internal relaxation through collisions, and it may be possible to generate new fragmentation pathways.
This invention comprises RF multipole ion guide configurations contained in regions of low and high pressure, as well as in regions of the pressure gradients. Multiple RF multipole ion guides are positioned end-to-end, and extend continuously between high and low pressure regions, and between low and high pressure regions. As discussed above, there are numerous functions that may be optimally performed at low pressure. In this invention, the RF multipole ion guide is configured to permit mass to charge selection in either a low pressure or high pressure region, or in a region of pressure gradient. Additionally, additional functions such as low pressure CID can be performed by operating within pressure gradients.
The present invention has a variety of advantages, including improving the RT characteristics of an RF/DC resolving quadrupole, improving the entrance beam profile for a TOF or other mass analyzer, decoupling CID processes from ion transport, and permitting new functionality within ion guides, as will discussed below. This invention also provides improved mass to charge isolation and selection. Resonant excitation isolation techniques are more selective using lower amplitudes at low pressure. Lower amplitudes reduce the power requirement, which saves complexity, cost and development cost. The present invention provides MS, MS/MS and MS/MSn mass analysis functions suitable for resolving RF/DC quadrupole mass filters, single or multiple ion mass-to-charge selection, axial DC acceleration CID ion fragmentation or resonant frequency excitation CID ion fragmentation.
Additionally, eliminating the electrostatic lenses between multipole ion guide assemblies increases ion transmission efficiency and allows ions to be efficiently directed forward and backward between quadrupole ion guide assemblies with high throughput. The functions of ion transfer, ion trapping and ion release are highly efficient. For example, ions can be released from one end of an ion guide assembly or segment simultaneously while ions are entering the opposite end of the ion guide assembly or individual segment. Due to this feature, an RF multipole ion guide receiving a continuous ion beam while operating in trapping mode can selectively release all or a portion of the ions located in the ion guide into another ion guide, ion guide segment or another mass analyzer that performs mass analysis on the released ions. Ion populations can be released from one end of an ion guide or ion guide segment operating in single pass or ion trapping mode simultaneously while ions are entering the opposite end of the multipole ion guide or individual segment. A segmented ion guide receiving a continuous ion beam can selectively release only a portion of the ions located in the ion guide into another multipole ion guide or other mass analyzer that performs mass analysis on the released ions. In this manner ions delivered in a continuous ion beam are not lost in between discrete mass analysis steps.
It is, therefore, an object of this invention to provide an improved multiple RF multipole configuration utilizing RF multipole ion guides that extend between various vacuum regions, with one RF multipole ion guide in the center held at reduced pressure, followed by another RF multipole ion guide held at elevated pressure. This permits the additional functionality, for example low pressure mass-to-charge selection followed by CID at elevated pressure.
It is another object of this invention to provide means for efficiently transporting ions from atmospheric pressure to vacuum, by means of RF multipole ion guides that extend between the high and low pressure regions, and to provide means of transporting ions through pressurized RF multipole ion guides, by means of one or more RF multipole ion guides that extend between a low pressure region and an elevated pressure region of the RF multipole collision cell.
It is, therefore, a further object of this invention to provide an improved means of transporting ions through pressurized RF multipole ion guides, by utilizing one or more RF multipole ion guides that extend between a low pressure region and an elevated pressure region of the RF multipole collision cell.